Your throwing rocks Narf.
Press the H key to cycle through the garnades you have. On the health, ammo, armo thingly to the very right you will notice it changing.
I have had many accidents with this and you will never forget the first time you blind yourself with a flash bang garnade or throw a rock instead of a shrapnel garnade.
Go for the Head shot if at all possible. Sometimes you get full armor from the bad dudes you kill.